The inventors have developed a 4.7 GB/DVD-RAM (hereinafter, DVD-RAM), which is an overwritable information recording medium used for data files and image files and have commercialized said DVD-RAM in the year 2000. In the original DVD-RAM put to practical use, a recording layer had a two-layer construction of SnTe/Ge—Sb—Te (see, for example, Japanese Patent Kokai (Laid-Open) Publication No. 2001-209970). This is a construction in which a crystallization improvement layer composed of SnTe having strong crystallinity is formed before a phase transformation layer composed of a high-speed crystallization material Ge—SB—Te (see, for example, Japanese Patent Publication No. 2584741) that has been conventionally used in a product thereof.
Moreover, the material of the recording layer of the DVD-RAM put to practical use includes Ge—Sn—Sb—Te (see, for example, Japanese Patent Kokai (Laid-Open) Publication No. 2001-322357). This is a material in which SnTe is added to GeTe and Sb2Te3. SnTe is a telluride whose crystalline structure is the same rock salt-type structure as that of GeTe, and therefore, SnTe is added to partially substitute for GeTe. Therefore, when this material was used, crystallization speed was enhanced without phase separation due to repeated recording. To ensure stability of the amorphous phase, addition concentration of the SnTe was carefully optimized. Thus, the present inventors have found that a practical recording layer is composed of a specific two-layer construction and is conveniently formed by materials to which specific additives are added.
Currently, 2-fold speed (22 Mbps) and 3-fold speed media are commercially available for DVD-RAMs. Moreover, 1-fold speed (36 Mbps) rewritable Blu-ray Discs (hereinafter, BD-RE) as recording media for high definition images were commercialized in 2004. Capacities of BD-REs are 25 GB and 50 GB. In recent years, increased data processing speeds have been required for media for datafile applications, and high-speed dubbing has been required for media for image file applications. Considering these requirements, further development is required for higher-speed recordable BD-REs and DVD-RAMs. Specifically, adaptation to 12-fold speed for BD-REs and 16-fold speed for DVD-RAMs is required.
16-fold speed corresponds to the linear speed at the outermost circumference of the medium when the rotation frequency of the drive motor is about 11000 rpm (the current approximate upper limit). For a recording method using constant angular velocity (hereinafter referred to as CAV), a 12-cm diameter medium provides a linear speed at the outermost circumference of the medium that is approximately 2.4 times larger than the linear speed at the innermost circumference. Therefore, development is required for CAV-mode BD-REs capable of recording/erasing favorably in the range of 5-fold speed to 12-fold speed, and for CAV-mode DVD-RAMs capable of recording/erasing favorably in the range of 6-fold speed to 16-fold speed.
For both BD-REs and DVD-RAMs, drastic improvement of crystallization speed of the recording layer material is essential for adapting to linear speeds several-times more than that of commercially available product. Accordingly, for example, increasing the concentration of SnTe in Ge—Sn—Sb—Te has been proposed, or material in which GeTe and Bi2Te3 are mixed, referred to as Ge—Bi—Te based material (see, for example, Japanese Patent Publication No. 2574325), has been proposed as an ultrahigh-crystallization-speed material.